ORIGIN AND SIGNIFICANCE OF SPINES 



51 



spinulosus,^^ Actinocrinus lobatus^^^ A. pernodostis, Oreaster 

 oc<jide)italis^ 0. gigas^ Retaster cribrosus, etc. 



The concentric laminae of growth in the Brachiopoda are 

 frequently differentiated into spinules; as in Sip1io7iotreta 

 unguieulata,^^ Schizamhon ti/jncalis^'^^ Spirifer jimbriatus^^^ 

 S. pseudolineatus,^^ iS. setigerus^^ CUothyris Royssii^^^ etc. 

 Other species show the differentiation of the radii into 

 spines; as Acanthothyris spinosa"'^ and A. Doderleini.^^ In 

 others the strong concentric lamingB passing over radii are 

 often infolded into spines; as in Atrypa spinosa.^^ 



Among the Mollusca innumerable examples could be cited 

 showing clearly the differentiation of various ornamental 

 features into spines. Some of these 40 



have already been discussed, but 

 may be referred to again in this 

 connection. Thus an illustration 

 of the passage of concentric laminae 

 into spines is shown in Avicula 

 sterna ^ and Anomia aculeata ^^ (fig- 

 ures 26 and 28) and Margaritiphora 

 jimbriata^ etc. Many species of 

 Gastropoda show the same types of 

 differentiation. The differentiation 

 of radiating lines or ridges into 

 spines is equally common, and is well 

 sliown in Spondylus (figures 12, 14, 

 30), and in Lima squamosus (figure 40). In most of these 

 cases the rib represents the progression of a fold in the 

 edge of the mantle, while the spine is a process of a con- 

 centric lamina, and is usually more or less flat or tubular. 

 Occasionally the rib becomes obsolescent, and is represented 

 by a row of spines ; as in some specimens of the gastropod 

 Crucibulum spinosum. When the radiating and concentric 

 ornaments are distinctly continuous, a reticuhite or cancellate 

 appearance is produced, and the points of intersection often 

 bear spines; as in Aviculopecten scabridus^ A. oriiatus^^ 

 Actinopteria Boydi^^ Pterinopecten spondylus,^^ etc. 



Figure 40. — Lima sr/ua- 

 7nnsiis. Natural size. To show 

 differentiation of radii into 

 spines. 



